Epoxy resins and epoxy prepregs including carbon fiber prepregs are known to be useful for aerospace applications. For such aerospace applications, it is important that the resulting prepregs have a high glass transition temperature (Tg) [for example greater than 160° C.] and a high char yield (for example having a char residue of greater than or equal to 35 percent [%] residue) for fire resistance.
For example, US 2011/0184091 discloses a hot-melt prepreg used in the aerospace industry employing (1) two or more epoxy resins, wherein one of the epoxy resins contains an oxazolidone ring structure, and (2) 4,4′-diaminophenylsulfone (DDS) as the curing agent. While the composition disclosed in US 2011/0184091 provides cured product useful in the aerospace industry, there is still room for improving the composition including providing a balance of properties such as a high Tg and a high char yield of the cured product made from the composition. In addition, US 2011/0184091 does not disclose the use of low viscosity epoxy resins such as divinylbenzene dioxide epoxidized cyclohexane tetraphenol (CHTP) or divinylbenzene dioxide (DVBDO) to prepare curable epoxy resin compositions.
Typically, a carbon fiber prepreg is produced via a known hot-melt process as shown in FIG. 1. The hot-melt prepreg process consists of film casting followed by prepreg formation and composites that are cured under a bag or in autoclave. Typical characteristics of an epoxy resin composition for a hot-melt process include solvent-free, high viscosity of the unreacted mixture (this characteristic is key in order to form a film) and latency of the chemistry during film formation. In addition, the “B-staging” phase of the process for making a prepreg is largely achieved by tuning the viscosity of the composition. Furthermore, the degree of “tack” at the B-stage ranges from low to high.
Typical compositions for hot-melt prepreg contain diluents such as cycloaliphatic and aliphatic epoxies, low viscosity liquid epoxy resins (LER) and other reactive diluents in order to tune the viscosity to the desired processability range. However, the use of reactive diluents can result in decreased thermal and mechanical properties, lower glass transition temperatures (Tg) and lower char yield for fire resistance.
What is needed in the field of preparing epoxy prepregs which include carbon fiber prepregs particularly for aerospace applications is epoxy resin compositions for preparing prepregs that exhibit a combination and balance of advantageous properties including for example processability, Tg, mechanical performance and char yield.